The announcement of abnormal road traffic conditions, such as a traffic jam, an accident, or a sudden traffic speed decrease is very important to limit the number of accidents on the road. On some highways, dedicated systems are in place for detecting some of these conditions, typically traffic jam conditions. These systems rely on different infrastructure means, such as speed sensors, video surveillance equipment, and information boards to announce abnormal traffic conditions. A problem is that such infrastructure means are expensive to deploy and to maintain. Furthermore they cannot react very quickly to sudden conditions, and they cannot react accurately to traffic conditions with a limited impact on the road.